I have a Dynaco Stereo 120 amp that has had the Bender ULT120 upgrade. It never had the TIP mod work done (though it is an older version that would call for it). I assume the Bender mod supplanted the Dynaco TIP mod.
It has been working fine until recently the right channel volume has reduced dramatically and is very scratchy.
I thought it might be the large output capacitor. These output caps were newly replaced when the Bender mod was done but I swapped the left and right channel caps and there was no difference. The left channel is clear and strong but the right channel is almost out.
I'm not well versed in electronics so I could use some help. Anyone have ideas where to start diagnosing the problem?
Thanks
Carl
It has been working fine until recently the right channel volume has reduced dramatically and is very scratchy.
I thought it might be the large output capacitor. These output caps were newly replaced when the Bender mod was done but I swapped the left and right channel caps and there was no difference. The left channel is clear and strong but the right channel is almost out.
I'm not well versed in electronics so I could use some help. Anyone have ideas where to start diagnosing the problem?
Thanks
Carl
I own and have upgraded a ST120. Not familiar with the bender modification. Hope it was replacing the dingbat PC15 regulator. Mine refused to work as specified with a replacent modern transistor which had too much gain. Instead I use a 69 v regulator consisting of a 72 v zener (6 x 12v) driving 5 parallel TIP147 darlington transistors in parallel, followed by .33 emitter resistors to balance current. All this on insulator board, with insulated heat tabs on a PentiumII heatsink.
Be aware there are five other electrolytic caps on each PC14 (channel driver) board. All of these are suspect unless replaced in the last 2000 hours of use. Everything from 5 uf to 250 uf. I use >3000 hour service life caps to prevent doing this again in my lifetime. I buy e-caps from Panasonic, nichicon, rubicon, vishay, kemet. Read the distributor selector table or datasheet for each cap you select. All 6 quality vendors sell a lot of 500 hour service life caps for experiments or those repair agencies that want the amp to fail again in 2-5 years. Note exact replacements are not available, caps now come in 1, 2.2 3.3 4.7 6.8 sequence with various multipliers.
Mark the board for + before removing, caps installed backward will vent the water out the top.
Wear safety glasses, solder splashes when it is liquid.
If you want a little better sound, the Apex AX6 has only 6 transistors, fewer ecaps, and much less crossover distortion when the amp is soft and cold. The PC14 sounds better after an hour at ~5 watts due to charge up of the 47 uf cap across the two bias diodes. I use 2 bias diodes plus a series schottky diode, parallel a 100 ohm pot to dial the cold idle current across the .51 ohm resistor down to 20 ma (or 10 mv).
https://www.diyaudio.com/forums/solid-state/236256-retro-amp-50w-single-supply-42.html
Same supply voltage. My AX6 board put out 72 watts for 5 seconds on Rhianna Shut Up & Drive. I have better heat sinks than OEM 1/8" angle aluminum.
Be aware there are five other electrolytic caps on each PC14 (channel driver) board. All of these are suspect unless replaced in the last 2000 hours of use. Everything from 5 uf to 250 uf. I use >3000 hour service life caps to prevent doing this again in my lifetime. I buy e-caps from Panasonic, nichicon, rubicon, vishay, kemet. Read the distributor selector table or datasheet for each cap you select. All 6 quality vendors sell a lot of 500 hour service life caps for experiments or those repair agencies that want the amp to fail again in 2-5 years. Note exact replacements are not available, caps now come in 1, 2.2 3.3 4.7 6.8 sequence with various multipliers.
Mark the board for + before removing, caps installed backward will vent the water out the top.
Wear safety glasses, solder splashes when it is liquid.
If you want a little better sound, the Apex AX6 has only 6 transistors, fewer ecaps, and much less crossover distortion when the amp is soft and cold. The PC14 sounds better after an hour at ~5 watts due to charge up of the 47 uf cap across the two bias diodes. I use 2 bias diodes plus a series schottky diode, parallel a 100 ohm pot to dial the cold idle current across the .51 ohm resistor down to 20 ma (or 10 mv).
https://www.diyaudio.com/forums/solid-state/236256-retro-amp-50w-single-supply-42.html
Same supply voltage. My AX6 board put out 72 watts for 5 seconds on Rhianna Shut Up & Drive. I have better heat sinks than OEM 1/8" angle aluminum.
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I sold a Stereo 120 a couple of years back.
They are robust amps...... HOWEVER they will not tolerate loose or removal of the RCA inputs while powered up.
Instant POP! and repairs are required.
A Mitstubishi DA-A10DC heavy brick amp 100/100W that I also owned was much more tolerant of mistakes and never gave problems.
They are robust amps...... HOWEVER they will not tolerate loose or removal of the RCA inputs while powered up.
Instant POP! and repairs are required.
A Mitstubishi DA-A10DC heavy brick amp 100/100W that I also owned was much more tolerant of mistakes and never gave problems.